[wasm][memory64] Fix typing of memory.size

memory.size returns in i64 if memory64 is enabled.
This CL fixes typing and adds a decoder test. Execution will be tested
and fixed in a follow-up CL.

R=manoskouk@chromium.org

Bug: v8:10949
Change-Id: I15818a6273b579d0faacec7f77dc813ae9ba218f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2632593Reviewed-by: Manos Koukoutos <manoskouk@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72132}

src/wasm/function-body-decoder-impl.h

@@ -3045,7 +3045,8 @@ class WasmFullDecoder : public WasmDecoder<validate> {
   DECODE(MemorySize) {
     if (!CheckHasMemory()) return 0;
     MemoryIndexImmediate<validate> imm(this, this->pc_ + 1);
-    Value* result = Push(kWasmI32);
+    ValueType result_type = this->module_->is_memory64 ? kWasmI64 : kWasmI32;
+    Value* result = Push(result_type);
     CALL_INTERFACE_IF_REACHABLE(CurrentMemoryPages, result);
     return 1 + imm.length;
   }

test/unittests/wasm/function-body-decoder-unittest.cc

@@ -5008,6 +5008,16 @@ TEST_P(FunctionBodyDecoderTestOnBothMemoryTypes, 64BitOffset) {
 #undef ZERO_FOR_TYPE
 }
 
+TEST_P(FunctionBodyDecoderTestOnBothMemoryTypes, MemorySize) {
+  builder.InitializeMemory(GetParam());
+  // memory.size returns i32 on memory32.
+  Validate(!is_memory64(), sigs.v_v(),
+           {WASM_MEMORY_SIZE, kExprI32Eqz, kExprDrop});
+  // memory.size returns i64 on memory64.
+  Validate(is_memory64(), sigs.v_v(),
+           {WASM_MEMORY_SIZE, kExprI64Eqz, kExprDrop});
+}
+
 #undef B1
 #undef B2
 #undef B3